Increasingly strict statutory regulations regarding admissible pollutant emissions of motor vehicles, in which internal combustion engines are disposed, make it necessary to keep the pollutant emissions during operation of the internal combustion engine as low as possible. This may be done firstly by reducing the pollutant emissions that arise during combustion of the air-fuel mixture in the respective cylinders. Secondly, in internal combustion engines use is made of exhaust gas after-treatment systems, by means of which the pollutant emissions produced during the process of combustion of the air-fuel mixture in the respective cylinder are converted into harmless substances. Especially in gasoline internal combustion engines, three-way catalytic converters are used as catalytic converters. A prerequisite of highly efficient conversion of the pollutant components, such as carbon monoxide (CO), hydrocarbons (HC) and nitrous oxides (Nox), is a precisely adjusted air-fuel ratio in the cylinders.
Furthermore, the composition of the mixture upstream of the catalytic converter has to exhibit a defined variation and so a purposeful operation of the internal combustion engine both with excess air and with an air deficiency is necessary in order to achieve filling and emptying of an oxygen store of the catalytic converter. With the storing of oxygen in the oxygen store nitrous oxides in particular are reduced, while with emptying of the oxygen store the oxidation is assisted and a deactivation of sub-regions of the catalytic converter by stored oxygen molecules is further prevented.
The oxygen store is inter alia also designed to store oxygen over a very short term and, where necessary, to bind or release this oxygen. The oxygen store comprises a surface store and a sub-surface store for the storage of oxygen.
From the technical manual “Internal Combustion Engine Manual”, edited by Richard von Basshuysen/Fred Schäfer, 2nd edition, June 2002, Friedrich Vieweg and Sohn Verlaggesellschaft mbH Braunschweig/Wiesbaden, pp. 559-561, a lambda closed-loop control operation and an associated trim closed-loop control operation for gasoline-operated internal combustion engines is known. By means of the lambda closed-loop control operation it is to be ensured that the pollutant components CO, HC and NOx are converted as efficiently as possible, namely in connection with the use of a catalytic converter in the form of a three-way catalytic converter.
The lambda closed-loop control operation comprises the modulation of a forced excitation upon a setpoint value of the air-fuel ratio in order to optimize the catalytic converter efficiency. In dependence upon the signal of a linear lambda sensor upstream of the catalytic converter an actual value of the air-fuel ratio is determined and so a system deviation is determined for the lambda controller, which takes the form of a PII2D controller and at the output of which a correction value for correcting a fuel quantity to be metered is determined. The fuel quantity to be metered that is corrected by means of the correction value is apportioned by the fuel injection valves into the combustion chambers of the cylinders.
The trim controller takes the form of a PI controller, which utilizes the signal of the sensor downstream of the catalyst that is subject to less cross sensitivity.
From the same technical manual, pp. 568 ff., it is known to monitor the catalytic converter. For this purpose, the oxygen storage capacity of the catalytic converter is utilized, which correlates with the hydrocarbon conversion in the catalytic converter. For a catalytic converter diagnosis an increased forced excitation is used. In a new catalytic converter with a relatively high oxygen storage capacity, these control oscillations are markedly damped and so the sensor signal downstream of the catalytic converter has only a low oscillation amplitude. An aged catalytic converter has markedly poorer storage properties, with the result that the oscillation upstream of the catalytic converter has a correspondingly greater impact on the exhaust gas sensor downstream of the catalytic converter. To monitor the catalytic converter, therefore, the signal amplitudes of the lambda sensors upstream and downstream of the catalytic converters are evaluated and then the quotient of the amplitudes is formed. This amplitude ratio is used to assess the conversion rate of the catalytic converter.
An increasingly frequent requirement is also to carry out monitoring with regard to possible nitrous oxide emissions and in particular also to estimate the NOx emissions.